1. Field of the Invention
The present invention relates to an apparatus for automatically assembling a plurality of semiconductor chips to provide a semiconductor device.
2. Description of the Related Art
FIG. 5 shows a general structure of a resin molded semiconductor device which includes a plurality of semiconductor chips. Reference numeral 51 denotes a lead frame, 52 an IC chip, 53 a transistor chip, 54 metal wire, 55 an adhesive, and 56 molded resin.
The semiconductor device, unlike a hybrid IC, employs lead frame 51 of a monolithic IC. A plurality of different or similar semiconductor chips, such as IC chip 52 and transistor chip 53, are bonded to an island of the lead frame 51. The electrodes of the semiconductor chips are connected to inner leads of the lead frame 51 by means of metal wires 54, thus providing the semiconductor device with a number of functions. This type of semiconductor device has been widely used because of the advantages: automatic fabrication is easily carried out, a good thermal conductivity is attained, efficient use of space is possible, and various kinds of packages are available.
The semiconductor chips are mounted on the island of the lead frame 51, typically, by means of adhesive 55. The adhesive 55 is, for example, electrically conductive paste, solder, gold foil (generally, an opposite surface of a semiconductor chip is plated with gold, and gold foil is rarely used).
FIG. 6 shows a conventional apparatus for automatically assembling the aforementioned semiconductor chips to provide a semiconductor device.
In a lead frame feed unit 61, lead frames are stocked in a magazine or stocked under a stacked state. The stocked lead frames are fed to a lead frame preheating unit 62 one by one and heated to a predetermined temperature. Subsequently, the lead frame is conveyed onto an XY-table 63. A semiconductor chip is fed from a semiconductor chip feed unit 64. The semiconductor chip is then aligned by an aligning unit 65. The chip is bonded to the island of the lead frame disposed on the XY-table. Prior to the bonding, an adhesive is applied to the mounting area of the island of the lead frame by means of an adhesive supply unit 66. A chuck transfer unit 67 is movable along a Y-axis, and transfers semiconductor chips from the semiconductor chip feed unit 64 through the aligning unit 65 onto the island of the lead frame disposed on the XY-table 63, thereby supplying the adhesive from the adhesive supply unit 66 to the island. The semiconductor chip feed unit 64 is so designed as to be able to feed a desired one of different or similar semiconductor chips arranged in the form of a chip tray or a wafer. The semiconductor chips are bonded, for example, in such a manner that similar chips are first bonded to all islands and then different chips are bonded. After bonding the chips to one lead frame, the XY-table 63 approaches a lead frame cooling unit 68 (63' denotes a range of movement of the XY-table 63), and the lead frame is transferred to the cooling unit 68. After the lead frame is cooled to a predetermined temperature, it is stored in a lead frame storing unit (e.g. a magazine) 69. A series of these operations are controlled by a control unit 70.
In general, however, as the adhesives employed to bond the semiconductor chip to the island, an electrically conductive paste or liquid solder has been used. When the conductive paste is used, it is subjected to a normal-temperature treatment and, subsequently, it is subjected to a curing treatment, thereby increasing a bonding strength. In the adhesive supply unit 66, the adhesive is put in a petri dish-like container. The adhesive is transferred from the container to a chuck 71 provided at the top of the chuck transfer unit 67 and then applied to the island of the lead frame. Thus, the range of movement of the XY-table must be wide, and a large space is disadvantageously required. On the other hand, the distance between both ends of the lead frame stage on the XY-table 63 is so short that an atmosphere necessary for manufacture or a non-oxidizing atmosphere cannot be satisfied. Consequently, surface oxidation of solder proceeds, and the wettability required at the time of bonding the chip is degraded, resulting in a low yield of manufactured products. Besides, regarding the productivity, if the XY-table 63 is not situated near the lead frame cooling unit 68 when the bonding of the semiconductor chip is completed, time is wasted to move the XY-table 63 to the vicinity of the lead frame cooling unit 68. Regarding the characteristics of the products, if one lead frame is mounted on the XY-table and a reducing or non-oxidizing atmosphere needs to be created, the structure of the entire apparatus becomes complex owing to the inadequate space. Unless this atmosphere is maintained, the characteristics are degraded due to bad wettability of solder and oxidation of lead frame. As a result, a step of removing the surface oxide must be provided in the subsequent mounting process. On the other hand, in the case where a reducing or non-oxidizing atmosphere is created, the semiconductor chip is exposed to a mixed gas (e.g. a mixture of hydrogen gas and nitrogen gas in a ratio of 1:9) of the reducing or non-oxidizing atmosphere until the bonding of the chips to one lead frame is completed. Some of the chips may be degraded in characteristic by the influence of hydrogen.
FIG. 7 shows in detail the semiconductor chip feed unit 64, aligning unit 65 and chuck transfer unit 67 in the conventional apparatus shown in FIG. 6.
The chuck transfer unit 67 is movable in the Y-direction by means of a ball screw 72. The amount of movement is controlled by the number of rotations (angle) of the ball screw 72. In the top of the chuck transfer unit 67, the chuck 71 is connected to a vacuum line. In the semiconductor chip feed unit 64, predetermined semiconductor chips 74, such as IC chips or transistor chips, are attached to a predetermined sheet 73. Though not shown, the semiconductor chip 74 to be picked up is detected by a TV camera, and the position of the chip 74 is corrected by an XY-table 75 so as to be picked up by the chuck 71. Thereafter, a pin 77 of a pushing unit 76 pushes one of the semiconductor chips, and the chuck 71 holds the pushed chip. The pushed chip is then transferred to the aligning unit 65. The chip is placed on an aligning table of the aligning unit 65. In order to bond the chip 74 to the island of the lead frame with high precision, the chip 74 is aligned with an aligning member 78.
It is difficult, however, to change the chuck 71 at the time of assembling the semiconductor chips. Therefore, it is necessary to treat semiconductor chips having different outer sizes by use of a single kind of chuck 71. If the chuck 71 is designed to hold the semiconductor chip having a small outer size, it cannot surely hold the semiconductor chip having a large outer size. In other words, deficient chucking occurs to provide a semiconductor device without the large mounted chip. As a result, the yield of products may be reduced. Although it is possible to detect the presence/absence of the semiconductor chip and to stop the operation of the apparatus if the chuck does not hold the chip, this decreases the operational efficiency of the apparatus and deteriorates the productivity. In addition, since the top of the chuck 71 is in contact with the electrode wiring layers of the semiconductor chip, the wiring surface is contaminated. In particular, a fine wiring pattern is damaged by contamination, resulting in a decrease in yield. It is also possible to use a recessed chuck so as not to contact the surface of the semiconductor chip. However, a plurality of semiconductor chips having different outer sizes must be treated by the single kind of chuck 71.
As has been described above, in the conventional semiconductor manufacturing apparatus, electrically conductive paste or liquid solder has been used as an adhesive. The adhesive is transferred by a chuck and supplied onto an island of a lead frame. In addition, the range of movement of the XY-table is wide, and efficient use of space is difficult. Thus, a reducing or non-oxidizing atmosphere is difficult to create, and the wettability of the semiconductor chip at the time of bonding is degraded. Consequently, the yield of products decreases and the quality of products deteriorates. On the other hand, if the reducing or non-oxidizing atmosphere is created, the structure of the apparatus is complicated.
Furthermore, the conventional semiconductor manufacturing apparatus is provided with only a single kind of chuck. If the chuck is designed to treat the semiconductor chip with a small outer size, it hardly holds or transfers the chip with a large outer size. If the apparatus is provided with a function of detecting the presence/absence of the semiconductor chip, the operational efficiency of the apparatus is lowered and the productivity is deteriorated.